By 2050 around 75% of all the world's population will be urban. Is this a good thing?

Population density, when done right, is a great tool to make people happier, give them more opportunities (social, economic, cultural, etc) and reduce their environmental footprint. A big part of it is that you can reduce the amount of pollution caused by transportation and housing, the two biggest resource sinks, with walkable neighborhoods and mass transit, as well as smaller dwellings (but the city becomes your living room and playground, so the actual "living area" can be much larger than for those living in some exurb in a McMansion...).